The radio spectrum is the range of the electromagnetic spectrum between 3 Hz and 3 THz (a frequency difference factor of a million). Because the radio spectrum is so valuable for many forms of communications, it is heavily coveted, used, and regulated.
Below 3 Hz the capacity to send information is very slow. Above 3 THz the earth’s atmosphere absorbs most of the energy, rendering transmission impossible until frequencies approach those of visible light (which are around 300 THz).
Countries establish national laws regulating the radio spectrum. Those laws are then coordinated through an international body, the International Telecommunications Union (ITU).
In 1947, the ITU defined nine bands for the radio spectrum, giving them impressively descriptive names. For example, the top six bands in the ITU hierarchy are called “High Frequency”, “Very High Frequency”, “Ultra High Frequency”, “Super High Frequency”, “Extremely High Frequency”, and “Tremendously High Frequency”. (I’m not making this up.)
Different frequencies exhibit very different characteristics. The high frequency bands can carry much more information (since they are higher frequency…). Furthermore, different frequencies exhibit different propagation properties: some can travel through water, some can diffract around large objects (like mountains), some can bounce off the ionosphere. The four bands between “High Frequency” and “Super High Frequency” are those competed for most by telecommunications services.
Other nomenclatures for the radio spectrum also exist. The IEEE has a competing set of designations. The EU / NATO / US have yet another nomenclature.
There are many ongoing ongoing policy negotiations around spectrum usages and ownership — often complicated by the rapid change of technology, which allows new approaches to using and sharing bandwidth (through technologies such as spread spectrum transmission, frequency reuse, dynamic spectrum management, frequency pooling, and cognitive radio).
Satellite communications present a new set of policy challenges. Low Earth Orbit (LEO) satellites can share bandwidth with terrestrial sources, so there are many guidelines to be decided to guarantee that satellites appropriately share spectrum with both ground-based and other satellite-based sources.